Method of making a low alloy steel



NOV- 10, 1970 A. F. Dx-:lfRETANA METHOD OF MAKING A LOW ALLOY STEELFiled May l5, 1967 2 Sheets-Sheet 1 IN VENTOR.

ANTHONY F. de RETANA 5, axngoaadwal ATTORNEY.

Nov. 10, 1970 A. F. DE RETANA 3,539,404

METHOD OF MAKING A LOW ALLOY STEEL Filed May l5. 1967 2 Sheets-Sheet 2INVENTOR.

ANTHONY F. de RETANA ATTR/VEY.

United States Patent O 3,539,404 METHOD OF MAKING A LOW ALLOY STEELAnthony F. DeRetana, Poland, Ohio, assignor to The Youngstown Sheet and'IJbe Company, Youngstown, Ohio, a corporation of Ohio Filed May 15,1967, Ser. No. 638,436 Int. Cl. C21d 1/02 U.S. Cl. 14S-12.4 4 ClaimsABSTRACT OF THE DISCLOSURE A low cost, low alloy steel containing notmore than about 0.25% molybdenum (Mo) in the presence of carbonitrideformers, carbide formers or both in relatively small quantities, as wellas a method of treating the steel to attain high yield strength (80,000p.s.i. or more) and good low temperature impact toughness (transversecharpy V-notch 15 ft. lb. at 50 F. or less).

BACKGROUND OF THE INVENTION The invention is directed to improvements inthe production of alloy steels low in Mo by reducing the amount thereofheretofore thought essential and provision of a novel method of treatingthe steel whereby the properties of strength and toughness heretoforeattained by the employment of relatively large amounts of Mo areimparted to it during the cooling stages to which it is subjectedfollowing a hot working or other mill process wherein it it heated to anelevated temperature, typically one in the neighborhood of 2100 F.

SUMMARY OF THE INVENTION A principal object of the invention thereforeis to provide for subjecting a low Mo steel of predetermined chemicalcomposition containing not over 0.25% Mo to a predetermined coolingoperation whereby when the steel finally emerges therefrom its yieldstrength, ten sile strength and impact transition temperature are withinoptimum ranges, for example, yield strength of approximately 80,000p.s.i. or more, tensile strength in the neighborhood of 100,000 p.s.i.and impact transition ternperature 50 F. or less.

It is well known that addition of Mo to steel in amounts approximating0.5% and upwards with about 0.002% of boron (B) present, results inproduction of steel of good hardenability, strength and toughness but asfar as is known these properties have not been attained to a like degreewith less than said quantities of Mo and B, and it is therefore furtherwithin contemplation of the invention to provide steel containing notmore than about 0.25 Mo and preferably 0.0005 to 0.0015% B with otheralloying elements in small amounts and to subject it to a novelprocessing whereby good yield and tensile strengths and low impacttransition temperatures are attained at a cost for the finished steelmaterially lower because of the lesser amounts of expensive alloyingelements used in its production than heretofore believed essential forsecurement of comparable results.

Other objects, purposes and advantages of the invention will hereinaftermore fully appear or be understood from the following more specicdescription in which reference will be had to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph based ondetermination of the eiects on transformation temperature of changes inthe cooling rate after heat processing of the low alloy steel of myinvention;

FIG. 2 is a graphic representation of the effect on 3,539,404 PatentedNov. 10, 1970 DESCRIPTION OF THE PREFERRED EMBODIMENT More particularlythe invention will now first be described with more detailed referenceto the several alloying elements employed in the steel and the amountsof each contemplated thereby and then to the processing to which thesteel containing them is subjected to attain the results set forth andsummarized in said graphs, it being understood that steel of any of theseveral specic analyses given is within the scope of the invention andmay be subjected to treatment in accordance with the method thereofutilizing any of several specifically slightly differing processingsteps as will hereinafter be more fully explained.

Thus, in general, the invention is directed and is in fact confined tosteel containing at least some of the elements within the range of thepercentages set forth in the following table in which, it will beunderstood, the difference between the totals of said elements and issubstantially only iron with perhaps very small percentages of sulfur,phosprorus and the like constituting impurities; it is thereforeunnecessary to include percentages of iron in each instance:

In addition to these elements, Ni and Cu may be added without detrimentto the physical properties and will improve the corrosion resistance asis well known by those versed in the art; it will of course beappreciated that inclusion of an element or elements in the foregoingtabulation is not intended to mean that each or all except C, Mn, Si,Mo, B and Ti are necessary in a given steel.

A number of samples of steels of slightly diiierent analysis yet allwithin said ranges and after treatment in accordance with my novelmethod, were subject to tests to determine factors on which theaforesaid graphs are based; two such steels hereinafter designated steelA and steel B respectively, substantially identical in analysis with theexception that steel A contained 0.37% chromium while steel B was freeof that element, contained the following in the amounts given:

More specifically, test pieces of these steels simulating plates ofgages yl, 1/2 and 5/e" respectively were heated to 2100 F., cooledslightly, and hotworked prior to being subjected to treatment inaccordance with the invention with the following results, thetemperature of the pieces at the initiation of the treatment being 1700cF. in all cases:

TABLE 2 Spray Spray Air cool cool Air cool e001 time, To rate, time, Torate,

Steel sec. F. Fjsee. min/see. F. F./seo.

e Plate:

A1 and B1 35 1, 050 18. 6 2. 40 750 1. 88

A2 and B2 27 1, 150 20. 4 3.20 750 2.00

A3 and B3. 2l 1, 250 2l. 4 3. 50 750 2. 18 2li" Plate:

A4 and B4 50 1, 050 13. 0 5. 00 750 1.00

A5 and 135---- 40 1, 150 13. 8 6. 00 750 1. 11

B6 25 1, 250 18.0 8. 00 800 0. 94 Plate:

A7 and B7. 100 1, 050 6. 5 6. 20 800 0. 60

A8 and B8... 70 1, 150 7. 9 7. 50 800 0. 75

A9 and B9 55 1, 250 8. 2 9. 00 800 0. 83

When the foregoing pieces, after treatment, were tested to determineyield strength and tensile strength, and some to determine in additionimpact transition temperature, the following results were observed:

TABLE 3 Impact Yield Tensile transition strength, strength, temperaturep.s.i. p.s.i. FJ l 91, 000 113, 000 91, 000 110, 000 se, 000 107, 00091, 000 108, 000 87, 00() 107, 500 se, 000 103, 000 97, 000 113, 000 11095, 000 110, 000 -120 88, 000 107, 000 -90 se, 000 100, 000 -90 82, 000s0, 00o 03, 000 e5, 400 se, 300 s0, 500 s0, 000 104, 500

1 size eharpy V-noteh, 5 ft. lbs.

Heating to 2100o F.-2200 F., in all instances precedes the predeterminedcontrolled cooling of the steel in accordance with the invention andthis may occur as an incident to other mill processes, for example asresidual heat imparted in the normal course of production as in asoaking operation. The steel is then mechanically deformed preparatoryto the subsequent specied cooling, and it is substantially immaterialwhat specific apparatus be employed for the spray cooling provided therates of cooling be not substantially different from those I have foundappropriate and so approximately within the limits of the treatments setforth in the foregoing Table 2.

The graph of FIG. l, based on the effect of various cooling rates in 1/2steel plate of analysis within the ranges of Table 1 and containing,more specifically, 0.21% Mo and 0.0015% B, demonstrates that the coolingrates within the scope of the invention, i.e., about 822 F./sec. duringthe spray cooling to a temperature above 1050J F. and about 0.7-2.2 F./sec. during subsequent air cooling confers a structure in the steelwhich is free of martensite and consists mainly of acicular ferrite andupper bainite.

The graphs of FIGS. 2 and 3 are designed to show the effects, plotted inincrements of increasing yield strength in p.s.i., of inclusion in thelow Mo steel of my invention of varying amounts of certain of thealloying elements heretofore mentioned; these results were derived fromtests of samples simulating 1/2" plate.

Hence within the scope of my invention is the provision of steel ofcomposition within the analysis of Table 1 and treatment thereof bycontrolled cooling from 1700 F. or higher initially at a relativelyrapid rate by spray cooling for a limited period followed by controlledslower cooling in air for a considerably longer one, all to produce lowalloy molybdenum steel of characteristics making it eminently suitablefor fabrication into articles of commerce.

I claim:

1. A method of treating a steel composition containing U13-0.22% carbon,0.69-l.28% manganese, 0.l5-0.40'% silicon, (L11-0.25 170 molybdenum,0.0005-0.0015% boron, 0.0100.050% titanium, about 0.015-.050% of atleast one element selected from the group consisting of columbium andvanadium, which method comprises:

(a) heating to about 2200 F.

(b) hot-working to about 1700a F.;

(c) spray-cooling from about 1700" F. to a range about 1250-1000'a F. ata rate of about 6.5-22 F. per second; and

(d) controllably air-cooling to SOO-750 F. at a rate of about 0.66-2.2F. per second, to thereby produce a steel product having amicro-structure of acicular ferrite and upper bainite and characterizedby the absence of martensite and by a yield strength of at least 80,000psi.

2. A method as dened in claim 1 in which the steel contains columbiumselected from said group in the proportion of about 0.0150.030%.

3. A method as defined in claim 1 in which the steel contains vanadiumselected from said group in the proportion of about 0.023-0.050%.

4. A method as defined in claim 1 in which the steel contains columbiumselected from said group in the proportion of about 0.0150.030% andvanadium selected from said group in the proportion of about0.023-0.050%.

References Cited UNITED STATES PATENTS 2,857,299 10/1958 Epstein et al14S-12.4 2,858,206 10/1958 Boyce et al 148-12 X 3,010,822 11/1961Altenberger et al. 148-36 X 3,102,831 9/1963 Tisdale 148-12 3,163,56512/1964 Wada 75-123 X 3,251,682 5/1966 Wada 75-123 X 3,290,183 12/1966Ohtake et al 148-12 3,330,705 7/1967 Madrzyk et al 148-12 FOREIGNPATENTS 28,241 12/ 1965 Japan.

OTHER REFERENCES Irani et al.: Quenched and Tempered Low-Carbon SteelsContaining Niobium or Vanadium, Journal of Iron & Steel Institute, July1966.

HYLAND BIZOT, Primary Examiner

